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# Thank Goodness for Gravity

Media Type:
Video

Running Time: 5m 46s
Size: 17.2 MB

or

Source: "Thank Goodness for Gravity"

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The boy featured in this video has concerns about spinning through space on Earth until he is introduced to a Foucault pendulum. He learns that French scientist Jean Bernard Léon Foucault used his knowledge of pendulums to demonstrate Earth’s rotation and how that movement is a steady, measurable motion rather than a dangerous one. KET adapted this video from the original produced by the Lexington Public Library in Lexington, KY.

Background Essay

We’re all moving. Everyday, because Earth spins on its axis, we wake up in the morning to a sunrise and go to bed in the evening after sunset.

Just how fast we’re moving depends upon our geographic location and the diameter of Earth at that point. For example, the rotational speed at the Equator is just over 1,000 miles per hour. The speed near the North and South Poles approaches 0 miles per hour.

The Foucault pendulum provided the first demonstration of this rotation that did not rely on astronomical observation. It was first performed in 1851 by French physicist Jean Bernard Léon Foucault. While working on another project, he noticed that if a pendulum is moved, it tends to keep swinging in its original direction. From this, he got the idea that he could demonstrate Earth’s rotation with a pendulum. He ran his first experiment in his cellar. With the success of that experiment, he went on to demonstrate Earth’s rotation publicly at the Paris Observatory and then at the Pantheon in Paris.

As you watch a Foucault pendulum for just a short time, its path will appear to be moving clockwise across the floor. But since there is no outside force making the pendulum rotate as it swings, and no other outside force interrupting that swing, it must be the floor itself that is rotating, while the path of the pendulum’s swing remains constant.

The apparent rotation of a Foucault pendulum is affected by two kinds of motion: twisting and traveling. Twisting is circular motion directly on an axis. Traveling is circular motion around an axis.

If a pendulum were placed at one of the Poles, the ground under the pendulum would twist around Earth’s axis. At the Equator, the ground under the pendulum would not twist, but travel eastward on Earth’s axis. Between the Equator and the Poles, the pendulum’s path is affected by a combination of the two motions.

Pendulums turn clockwise in the Northern Hemisphere and counterclockwise in the Southern Hemisphere.

To learn more about pendulums and how they work, check out Experimenting with a Pendulum and Virtual Pendulum.

Discussion Questions

• What does Foucault’s pendulum prove?
• Why don’t people and other objects fly off Earth if it is rotating so fast?
• How does the Foucault pendulum demonstrate the Earth’s rotation?

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